The present invention relates to electrochemical cells utilizing a non-aqueous gel polymer electrolyte with an intercalation cathode, and more particularly, to electrochemical cells utilizing a non-aqueous gel polymer electrolytic system, an intercalation cathode and a magnesium anode.
Rechargeable, high energy density electrochemical cells of various kinds are known. Such cells usually consist of a transition metal oxide or chalcogenide cathode-active material, an anode-active alkali metal or alkali metal intercalation compound, and an electrolyte solution containing a dissolved alkali-based salt in an aprotic organic or inorganic solvent, or polymer electrolyte.
Theoretically, a rechargeable cell is capable of charging and discharging indefinitely, however, in practice such performance is unattainable. The degradation mechanisms of the various anodes, cathodes and electrolytes are complex and are known to those skilled in the art.
Two basic types of cathodes are appropriate for a battery system that is rechargeable at ambient temperatures. A liquid cathode can be used, allowing reactions to take place with facility. Liquid cathodes are also advantageous in that thin films or crusts forming on the surface of the cathode tend to crack, such that the cathode activity remains high over the course of the cycling. The mobility of the cathodic material is a liability, however, in that contact with the anode short-circuits the cell. Thus, an electrochemical cell with a liquid cathode requires protective, insulating films on the anode.
A solid cathode must be insoluble in the electrolyte, and must be able to absorb and desorb a charge-compensating ion in a substantially reversible and fast manner. A prime example of a solid cathode of this variety is an intercalation cathode. Intercalation chemistry focuses on the insertion of ions or neutral molecules into an inorganic or organic matrix. In a typical intercalation cathode, cations dissolved in the electrolyte solution are inserted into the inorganic matrix structure.
A group of intercalation materials of particular importance is called Chevrel-phase materials, also known as Chevrel compounds. Chevrel compounds contain an invariant portion consisting of molybdenum and a chalcogenxe2x80x94sulfur, selenium, tellurium, or mixtures thereof. The invariant portion is generally of the formula Mo6Tn, where T represents the chalcogen and n is usually about 8. The unique crystal structure of Chevrel-phase materials allows the insertion of one or more metal ions in a reversible, partially reversible, or irreversible manner. The stoichiometry of the intercalation compound can be represented as MxMo6Tn, where M represents the intercalated metal and x may vary from 0 (no intercalated metal) to 4 or less, depending on the properties of the particular metal.
The intercalation of metal ions into the Chevrel compound releases energy. Since the process is partially or fully reversible, these compounds are particularly suitable as electrodes in electrochemical cells. For example, lithium, the predominant intercalation ion, can be removed from the Chevrel compound by the application of electrical energy. The energy is released as electrical energy upon reintercalation.
The cathode-active material in the high energy density, rechargeable electrochemical cells must be paired with a suitable anode-active material, which is most commonly made of an active metal such as alkali metals. However, the performance of a particular anode-cathode couple is strongly influenced by the nature of the electrolyte system. Certain non-aqueous electrolytes are known to perform well with a particular anode-cathode couple and be ineffective or significantly less effective with other anode-cathode couples, either because the electrolyte solution""s components are not stable or because the solutions components degrades during cycling active electrodes. As a result, much of the prior art relates to the cathode-active material, the anode-active material and the electrolyte not only as independent entities, but also as units within an appropriate battery system.
U.S. Pat. No. 4,104,451 to Klemann et al., discloses reversible batteries with an alkali metal anode, a chalcogenide cathode, and organometallic alkali metal salts in organic solvents as the electrolyte system. Non-aqueous electrolyte systems containing alkali metal salts of boron or aluminum anions based which also contain organic groups are disclosed.
Organoborate salts of alkali metals represented by the formula 
are disclosed in U.S. Pat. No. 4,511,642 to Higashi et al., wherein R1-R4 are organic radicals selected from the following groups: alkyl, aryl, alkenyl, cycloalkyl, allyl, heterocyclic, and cyano, and M+ represents an alkali metal ion.
U.S. Pat. No. 4,139,681 describes cells containing electrolytically active metal salt complexes having the formula ZMRnXi, wherein Z is a metal from a group containing aluminum, the Rs are specified haloorganic radicals, the Xs are selected from various halides, alkyls, aryls, alkaryls and aralkyls. M is specified to be an alkali metal, with lithium being the preferred embodiment.
U.S. Pat. No. 4,542,081 to Armand et al., describes solutions for the constitution of solid electrolyte materials of electrochemical generators. The compound is of the formula
(Rxe2x80x94Cxe2x89xa1C)4Zxe2x88x92, M+
in which Z is a trivalent element capable of entering into 4-coordination, such as aluminum, and R represents groups which are non-proton donors. M is specified to be an alkali metal.
The prior art described above, including U.S. Pat. Nos. 4,104,451, 4,511,642, 4,139,681 and 4,542,081, specifies that M is an alkali metal. The use of an alkaline earth metal anode such as magnesium would appear disadvantageous relative to the use of an alkali metal such as lithium because alkali metal anodes are much more readily ionized than are alkaline earth metal anodes. In addition, on recharge the cell must be capable of re-depositing the anode metal that was dissolved during discharge, in a relatively pure state, and without the formation of deposits on the electrodes.
However, there are numerous disadvantages to alkali batteries. Alkali metals, and lithium in particular, are expensive. Alkali metals are highly reactive. Alkali metals are also highly flammable, and fire due to reaction of alkali metal with oxygen or other active material is extremely difficult to extinguish. Lithium is poisonous and compounds thereof are known for their severe physiological effects, even in minute quantities. As a result, the use of alkali metals requires specialized facilities, such as dry rooms, specialized equipment and specialized procedures.
In contradistinction, magnesium metal and aluminum metal are easy to process. The metals are reactive, but undergo rapid passivation of the surface, such that the metals exhibit highly stable behavior. Both magnesium and aluminum are inexpensive relative to the alkali metals.
U.S. Pat. No. 4,894,302 to Hoffman et al., discloses an electrochemical cell having an intercalation cathode, an alkaline earth anode, and a non-aqueous liquid electrolyte containing an organic solvent and an electrolytically active, organometallic alkaline earth metal salt represented by the formula 
wherein Z is boron or aluminum; R1-R4 are radicals selected from the following groups: alkyl, aryl, alkaryl, aralkyl, alkenyl, cycloalkyl, allyl, heterocyclic alkyl, and cyano; and M represents an alkaline earth metal such as magnesium. The radicals can be inertly substituted with substituents that have no detrimental effect upon the electrolytic properties of the electrolyte composition with respect to effectiveness in an electrochemical cell, such as halogenated or partially halogenated derivatives of the above groups. While exhaustive care is taken to disclose a broad range of organic radicals and halogenated organic radicals, bonding the metallic species of the anion (Z) to another inorganic species is not considered.
U.S. Pat. No. 5,491,039 describes a solid, single-phase electrolyte containing a solid polymeric matrix and an organometallic ion salt represented by the formula
Mc(ZRn) 
wherein Z is boron, aluminum or titanium; Rn are various subsituted or unsubsituted organic radicals; M is lithium, sodium, potassium, or magnesium, c is 1 or 2, and n is an integer from 1 to 6. As in U.S. Pat. No. 4,894,302, a broad range of organic radicals including halogenated organic radicals is disclosed, but the bonding of the metallic species of the anion (Z) to another inorganic species is not reported. In all cases, metallic species Z is bonded to a carbon atom. More specifically, the bonding of the metallic species of the anion (Z) directly to a halogen is not disclosed. It must be emphasized that this is of particular significance in light of the fact that U.S. Pat. No. 5,491,039 teaches an extremely broad range of radicals that may be appropriate for attaching to the metallic species of the anion.
Both U.S. Pat. No. 5,491,039 and U.S. Pat. No. 4,894,302 disclose electrochemical cells having an alkaline earth anode such as magnesium. For commercial application, however, such magnesium batteries must be essentially rechargeable and must have a reasonable shelf life. Sustaining a voltage of 1.5 volts is problematic or impossible with the usual intercalation cathodes and electrolytes according to prior art. Magnesium batteries operating at 1.5 volts are particularly prone to electrolyte decomposition and to encrustation/passivation of both electrode surfaces.
In our co-pending parent application, Ser. No. 09/419,940, a new type of electrolyte for electrochemical cell was disclosed. The general formula of the electrolyte is Mxe2x80x2+m (ZRnXq-n)m in which: Mxe2x80x2 is selected from the group consisting of magnesium, calcium, and aluminum; Z is selected from the group consisting of aluminum, boron, phosphorus, antimony and arsenic; R represents radicals selected from the following groups: alkyl, alkenyl, aryl, phenyl, benzyl, and amido; X is a halogen (I, Br, Cl, F); m=1-3; and n=0-5 and q=6 in the case of Z=phosphorus, antimony and arsenic, and n=0-3 and q=4 in the case of Z=aluminum and boron. The above-mentioned co-pending application disclosed the compatibility of this electrolyte type with Mg and a magnesium-Chevrel intercalation cathode of the form
CuxMgyMo6S8 
to provide the basis for the production of a viable, rechargeable magnesium battery with a nominal voltage exceeding 1.5 volts.
Special interest is currently being focused on the use of solid polymeric electrolytes in advanced battery systems. The substitute of liquid electrolyte with solid gel polymeric electrolyte provides advantages in terms of safety, design flexibility and simplicity of production equipment and process operation.
Gels based on Li, Na and K ions are well known in academic literature and some of them have seen some extent of commercial implementation. By sharp contrast, very little work has been performed on gels based on Mg, Ca and Al ions. The relative paucity of work on Mg, Ca and Al gels may be attributed to several physical properties. The lattice energy of salts of Mg, Ca and Al is usually very high relative to those of salts of the alkali metals. Hence, it is extremely difficult to identify a polymer matrix that is polar enough to effectively cause ionic dissociation of these salts. It must be emphasized that the solubility of most of these salts is very low, even in water and in other high dielectric mediums, such that the possibility of finding a polymeric medium that can dissolve them with reasonable ionic dissociation appears remote. In addition, the ions of Mg, Ca, and Al are small and multivalent, such that the ions are characterized by extremely high charge densities, which tend to severely restrict ion mobility in solution. The transference number of such ions is expected to be less than 0.5 and the electrical conductivity of the solutions of such ions appear to be insufficient for the inventive electrolyte systems.
It must be further emphasized that the liquid electrolyte taught by our above-referenced co-pending parent application is based on a delicate equilibrium state in the solution. The electrochemical reactivity of the electrolyte is strongly dependent on the solvent structure and polarity. For example, the most extensively tested ethereal solvent in the above-referenced liquid electrolyte systems is tetrahydrofuran (THF). The use of very similar ether such as 2Me-THF, which differs from THF only by the addition of a single methyl group, was investigated. This seemingly minor difference has only a small effect on the polarity of the solvent, but in the inventive electrolyte system, such a difference was enough to cause a segregation of the complex salt and a loss in electrochemical reactivity. This example manifestly demonstrates that which is known on purely theoretical grounds, namely, that the solvent medium critically influences the structure and reactivity of the electrolyte complex salt.
Therefore, it would be surprising to find a polymer matrix in which the inventive electrolyte system will be soluble and compatible in such way that the complex will not segregate or react with the polymer matrix and the electrochemical reactivity will maintain.
Prior to the liquid electrolyte system taught by our above-referenced co-pending parent application, the only known type of electrolyte system in which magnesium could be electrochemically deposited and dissolved reversibly was a Grignard reagent in an ethereal solution. For example, Liebenow (Electrochimica Acta, Vol 43, Nos. 10-11, pp. 1253-1256, 1998) prepared a gel polymer based on ethyl magnesium bromide in tetrahydrofuran solution with polyethylene oxide as the polymeric matrix and demonstrated the ability of the system to reduce and oxidize magnesium. However, while gel polymers based on Grignard reagents may be of scientific interest, they cannot be applied to commercial battery technologies. The extremely poor anodic stability of gel polymers based on Grignard reagents precludes their use in such solid gel electrolytes. Moreover, the Grignard reagents are extremely flammable, corrosive and dangerous reduction materials.
Thus, there is a widely recognized need for, and it would be highly advantageous to have, a solid polymeric non-aqueous electrolyte that allows the production of an all solid, practical, rechargeable battery which would be more safe, clean, efficient and economical than rechargeable batteries known heretofore. It would be of further advantage if such an electrolyte would be based on magnesium, calcium, or aluminum, which are inexpensive and abundant raw materials.
The present invention is a new type of solid gel electrolyte for use in electrochemical cells. The properties of the solid gel electrolyte include high conductivity and an electrochemical window that can exceed 2.2V vs. Mg/Mg+2. The use of the electrolyte in an appropriate cell promotes the substantially reversible deposition of the metal and a reversible intercalation process at the cathode material.
According to the teachings of the present invention there is provided a solid, gel type non-aqueous electrolyte for use in an electrochemical cell, the electrolyte including: (a) at least one polymer compound; (b) at least one organic solvent, and (c) at least one electrolytically active salt represented by the formula:
Mxe2x80x2(ZRnXq-n)m 
in which: Mxe2x80x2 is selected from the group consisting of magnesium, calcium, and aluminum; Z is selected from the group consisting of aluminum, boron, phosphorus, antimony and arsenic; R represents radicals selected from the following groups: alkyl, alkenyl, aryl, phenyl, benzyl, and amido; X is a halogen (I, Br, Cl, F); m=2-3; n=0-5 and q=6 for Z=phosphorus, antimony, and arsenic, and n=0-3 and q=4 for Z=aluminum and boron, wherein the polymer compound, organic solvent, and electrolytically active salt interact to form a non-aqueous electrolyte having a solid, gel type structure.
According to another aspect of the present invention, the solid, gel type non-aqueous electrolyte is incorporated into an electrochemical cell further including a metal anode and an intercalation cathode.
According to yet another aspect of the present invention, there is provided a non-aqueous electrolyte for use in an electrochemical cell, the electrolyte including: (a) at least one organic solvent, and (b) at least one electrolytically active salt represented by the formula:
Mxe2x80x2(ZRnXq-n)m 
in which: Mxe2x80x2 is selected from the group consisting of magnesium, calcium, and aluminum; Z is selected from the group consisting of aluminum, boron, phosphorus, antimony and arsenic; R represents radicals selected from the following groups: alkyl, alkenyl, aryl, phenyl, benzyl, and amido; X is a halogen (I, Br, Cl, F); m=2-3; n=0-5 and q=6 for Z=phosphorus, antimony, and arsenic, and n=0-3 and q=4 for Z=aluminum and boron.
According to yet another aspect of the present invention, the non-aqueous electrolyte is incorporated into an electrochemical cell further including a metal anode and an intercalation cathode.
According to further features in preferred embodiments of the invention described below, Z is aluminum.
According to further features in preferred embodiments of the invention described below, Mxe2x80x2 is magnesium.
According to further features in preferred embodiments of the invention described below, Mxe2x80x2 is calcium.
According to further features in preferred embodiments of the invention described below, the electrolytically active salt is Mg[butylAlCl3]2.
According to further features in preferred embodiments of the invention described below, the electrolytically active salt is Mg[butylethylAlCl2]2.
According to further features in preferred embodiments of the invention described below, Mxe2x80x2 is selected from the group consisting of magnesium and calcium, Z is aluminum, R represents at least one type of alkyl radical, and m is 2.
According to further features in preferred embodiments of the invention described below, the organic solvent contains tetraglyme.
According to further features in preferred embodiments of the invention described below, the organic solvent contains tetrahydrofuran.
According to further features in preferred embodiments of the invention described below, the polymer compound serves as a matrix.
According to further features in preferred embodiments of the invention described below, the polymer compound is selected from the group consisting of PVdF, PEO, and PVC.
According to further features in preferred embodiments of the invention described below, Mxe2x80x2 is selected from the group consisting of magnesium and calcium, Z is aluminum, R is at least one type of alkyl radical, and m is 2.
According to further features in preferred embodiments of the invention described below, the intercalation cathode is a Chevrel-phase intercalation cathode.
According to further features in preferred embodiments of the invention described below, the Chevrel-phase intercalation cathode is represented by the formula
CuxMgyMo6S8 
wherein 1xe2x89xa7x greater than 0 and 2xe2x89xa7y greater than 0.
According to further features in preferred embodiments of the invention described below, the metal anode is magnesium.
According to further features in preferred embodiments of the invention described below, the polymer compound is selected from the group consisting of PVdF, PEO and PVC.
According to further features in preferred embodiments of the invention described below, the solvent is selected from the group consisting of THF and tetraglyme.
The present invention successfully addresses the shortcomings of the presently known solid gel electrolytes and provides the basis for the production of a viable, rechargeable battery based on magnesium, calcium, and aluminum, and having a nominal voltage exceeding 1.5 volts.